1. Field of the Invention
This invention relates to a combustion air trim control method and apparatus and more particularly, to an actuator for automatically adjusting the ratio between the flow of air and the flow of fuel to the burner of combustion apparatus.
2. Description of the Prior Art
In order to attain optimum fuel economy in the operation of a boiler, it is well known that the air/fuel ratio must be continuously adjusted in order to correct for changes in fuel heat values, oil viscosity, gas density, variations in air temperature and humidity, burner condition, fuel temperature changes, fuel pressure changes, linkage wear, and other factors. It has been found that one of the requirements for achieving optimum fuel economy is controlling the amount of excess air in a boiler. It is a common practice to operate boilers with 15% to 35% more air than is actually required. Excess air flow of this degree reduces flame temperature and carries usable heat out of the process.
A boiler operation must be "continuously tuned" by adjustments to the air/fuel ratio to compensate for changes in the above factors effecting optimum combustion. A known method of continuously adjusting the air/fuel ratio in a combustion process is by interconnecting the fuel control device with the air flow control device by a mechanical linkage.
U.S. Pat. Nos. 4,157,238 and 4,264,297 disclose an actuator which is adjustable to provide the desired air to fuel ratio throughout the operating range of the combustion apparatus. The disclosed linkage utilizes a jackshaft system for interconnecting the fuel valve with the damper. The interconnection also includes variable linkage means between the jackshaft and one of the flow control devices, either the fuel valve or the damper. The variable linkage responds to a control signal to change the calibration between the control devices in order to compensate for changes in the operating conditions enumerated above. The variable linkage means includes a lever having a length adjustable by a piston cylinder assembly. This arrangement permits changes to be made in the radius length of the lever arm. In addition, a link connecting the jackshaft through the lever arm to one of the flow control devices is also changeable in length. In this manner, the calibrated linkage relationship between the fuel valve and the damper is adjustable to compensate for changes in the operating conditions.
One of the problems encountered with the variable length-type linkage connections between a jackshaft and an air or fuel flow control device is an incomplete response or a non-response to the signal for actuating a change in the length of a plurality of levers and connecting links. When a change in the calibration between the fuel valve and the damper is required, unless the precise changes in the lengths of the adjustable interconnecting levers and links is made simultaneously, the recalibration will not be successfully completed. For example, if the length of one lever is changed and the corresponding change in the length of a connecting link to one of the flow control devices is not made, the system, rather than being tuned, becomes more inefficient. This can result in the air/fuel ratio being set too low resulting in incomplete combustion with unburned fuel carried away in the flue gases or too high resulting in excess air flow with heat being carried away in the flue gases.
While it has been suggested by the prior art devices to continuously adjust combustion apparatus to maintain an optimum air/fuel ratio by recalibrating the interconnecting linkage between the fuel flow and air flow control devices, the known control systems utilize a plurality of component parts which are independently controlled. They are subject to malfunction if the required adjustments to the components are not made. Therefore, there is need for combustion air trim control apparatus having a mechanical linkage that interconnects the fuel and air flow control devices in a manner which efficiently and reliably adjusts the linkage connection in response to a rapid change in the boiler operating conditions.